1. Field of the Invention
This invention relates generally to a multiplier circuit for generating a ratio or product of two or more input signals and, more particularly, to a CMOS sensor circuit that acts as an analog divider, multiplier and ratiometry circuit to provide a ratiometric output signal that is insensitive to process parameters and temperature variations on the circuit.
2. Discussion of the Related Art
Consumer demand for improved vehicle safety has caused several vehicle manufacturers to develop vehicle yaw rate control systems. The yaw rate for a vehicle is the angular rate of rotation about a vehicle's vertical axis. In other words, it is a measure of the turning of the vehicle to the left or to the right. A vehicle yaw rate control system compares the driver's desired turning rate to the actual turning rate of the vehicle, and provides a continuous feedback to maintain the vehicle directed towards the driver's desired heading. For example, if the right drive tire of the vehicle is on ice and the left drive tire is on asphalt, the vehicle will tend to rotate (yaw) towards the right even though the driver is attempting to maintain the steering of the vehicle in a forward direction. Thus, the control system would provide control signals to adjust wheel torque for the appropriate wheel or wheels to maintain the desired steering direction. The system would include a steering wheel angle sensor that provides a signal indicating the driver's desired turning rate, and a yaw rate sensor to measure the actual turning rate of the vehicle. The two input signals, as well as lateral acceleration, are used by the yaw rate control system to determine whether the vehicle is heading in the direction that the driver desires. An example of a yaw rate control system is described in Zarabadi, Seyed R. et al., "An Angular Rate Sensor Interface IC," IEEE 1996 Custom Integrated Circuits Conference, May, 1996, pp. 311-314.
In certain sensor systems, such as a vehicle yaw rate sensor system, it is necessary to provide a circuit that produces an output signal which is the ratio or product of the system's main power supply voltage and a temperature voltage signal from a temperature compensation circuit. When the circuit multiplies the power supply signal and the temperature voltage signal, its output is used as a reference to a closed amplitude loop to produce another output signal to provide a system output which is ratiometric to the power supply voltage, while canceling the sensor temperature sensitivity. If a circuit exhibits temperature, fabrication and component variance sensitivities, then some sort of calibration and trimming have to be incorporated to cancel these sensitivities over the life of the product. Calibration and trimming are expensive because they require a high equipment investment and significantly increase the product's test cost.
There are many known designs of metal oxide semiconductor (MOS) divider/multiplier/ratiometry circuits. A voltage divider/multiplier/ratiometry circuit is a versatile circuit that is used to generate a ratio/product of two or more signals. All of the known MOS circuits either do not implement an exact function or exhibit temperature and process sensitivity. The only multiplier known which implements the exact function and yet is insensitive to temperature and process variations is the well known bipolar Gilbert multiplier. The Gilbert multiplier is based on the translinear principle, and is widely used in many types of discrete multipliers. However, because the Gilbert multiplier uses bipolar devices which do not lend themselves to inexpensive standard high density CMOS processes, Gilbert Multipliers have disadvantages and are not practical for certain applications, such as yaw rate control system in a vehicle.
What is needed is an MOS divider/multiplier/ratiometry circuit that is insensitive to process parameters and temperature changes, and can be implemented in a yaw rate control system. It is therefore an object of the present invention to provide such a circuit.